In many industrial applications, an apparatus that is designed to operate at cryogenic temperatures is located within an insulated container to minimize heat leakage from the ambient to the apparatus.
An example of an apparatus that has operational temperature requirements is a cryogenic distillation apparatus in which air is compressed, purified and then cooled to a temperature at or near its dew point for distillation in one or more distillation columns to separate lighter components such as nitrogen and argon from heavier components such as oxygen. The incoming air is cooled against product streams such as nitrogen and oxygen within a main heat exchanger.
Another example is a device for liquefying natural gas whereby gas from a high pressure pipeline is expanded, cooled and condensed to produce a liquefied natural gas (LNG) product.
In order to maintain the low temperatures required for such cryogenic operations, the equipment can be placed in a container known as a cold box. Such a container operates at a positive pressure, that is, the container is not sealed to the ambient environment. Bulk fill insulation, ordinarily in particulate form, is introduced into the container to provide insulation. Such bulk fill insulation, for example perlite, inhibits both convective and radiative heat transfer and constrains the heat transfer occurring through conduction.
A minimum insulation thickness is required to prevent excessive heat leakage. Typically, the container may be fabricated from a carbon steel material, which may not be suitable for exposure to cryogenic temperatures. However, a minimum insulation thickness is required to avoid brittle failure of the container walls and structural supports. As can be appreciated, the lower the thermal conductivity of the insulation, the smaller the minimum thickness of insulation, and the smaller the container due to less insulation.
While current containers exposed to cryogenic operations provide protection from external corrosion, additional protection from external fires would be desirable, particularly for cold boxes installed in a marine environment. Additionally, secondary leakage protection would be desirable for a location with limited space, which includes marine environments.
Currently, cold boxes containing cryogenic processing equipment provide adequate heat leakage properties and provide an inert environment to preserve the equipment. However, the current state of cold box equipment fails to provide any significant protection from external fires or secondary leakage protection, both of which are particularly valuable for situations where plot space is very limited, such as a marine environment.